Disposable, absorbent articles such as diapers, incontinence articles, sanitary towels, training pants and the like are well known in the art.
Significant effort has already been spent against assessing the performance of such articles both with respect to wetting of outer garments (leakage) and with respect to wetting or lack thereof of the skin of the wearer.
To assess the condition of the skin of the wearer, the moisture content of the uppermost skin layer, the stratum corneo, is of critical importance, and many reports refer to the evaluation of such articles under in-vivo conditions.
Elsner et al. provides a comprehensive overview of such methods in "Bioengineering of the skin: Water and the Stratum Corneum", CRC Press, 1994. The most relevant methods are the "Transepidermal Water Loss" (often abbreviated TEWL) measuring the moisture evaporation from the skin; methods to measure the electrical properties like capacitance, impedance, or conductance of the skin, which depend strongly on the moisture content, such as with the CORNEOMETER or NOVA or other instruments. Also, further methods applying conventional chemical analysis tools like IR or NMR spectroscopic or magnetic resonance imaging are referred, too, however, have so far not found broad application.
The in-vivo methods have in common, that they asses directly the condition of the skin of the wearer of an absorbent article either under real in-use loadings or possibly with artificially loaded articles, which are for example worn on the forearm of a test person for a certain period.
For all these methods, the comparison of absorbent articles for development purposes is cumbersome. Apart from the fact of needing test persons as such, these persons have individual factors--such as varying reaction to certain room conditions as temperature or relative humidity--all contributing to a large variability of the test results. In order to still get meaningful data, the number of test persons must be increased--again increasing the effort.
Hence, significant effort has already been put against evaluating absorbent articles and structures under reproducible and easy to execute laboratory conditions, whereby generally the human skin is replaced by standardised fluid pick-up filter paper. Essentially, these methods are based on the "capillary rewet" principle, whereby a test sample is loaded with a certain amount of test fluid, such as synthetic urine. After a certain time such as to allow for equilibration and preferably under a certain pressure, the pick up filter paper as "skin replacement" is placed on top of the surface of the loaded structure for a certain time, under a certain pressure. The pick-up filter paper is well defined such as by porosity, basis weight, or absorbency. Due to the capillary suction power of its pores, it is sucking up readily available moisture (i.e. "free" moisture not being bound such as through superabsorbent materials or in smaller pores that the pick-up paper) from the surface of the test specimen and the weight increase is a measure for the "rewet" performance of the absorbent article.
Optionally, this test procedure can be combined with other fluid handling evaluation protocols, for example a "post-acquisition-rewet-test" indicates, that during the first part of the combine protocol the fluid acquisition behaviour of the test specimen is studied, whereas the rewet assessment in then carried out in the second part of the test.
A big number of such tests have been described in the public, such as in WO 93/02 188 (Guidotti et al.); EP-A-0 039 974 (Mullane); EP-A-0 278 601 (Kobayashi); or EP-A-0 539 703 (Hanson).
Another approach to assess the performance of such articles has been proposed by Lask et al. in EP-B-0 312 919, whereby the surface moisture e.g. of an absorbent article is correlated to the reflection and scattering of a light beam.
However, advanced core designs have resulted in "dryer and dryer" products, and the differentiation between "good" and "better" products has become increasingly difficult, if not impossible with these conventional methods. Still, both in-vivo measurements as well as comments of users of such articles clearly indicate, that there is a need for further discerning various products or designs to further improve the performance of such articles, and in particular to reduce skin hydration.
In addition, recent work indicated, that it not only the capillary fluid transport from the loaded article back to the skin of the wearer is impacting on the condition of the skin, but that other factors such as sweating under occlusive conditions can have very negative impact on the condition of the skin.
Hence it is been an object of the invention to provide a better tool for distinguishing well performing absorbent articles under reproducible laboratory conditions.
It is a further object of the invention to provide such a tool not only for capillary fluid transfer conditions, but also for other moisture transfer mechanisms, such as when sweating under occlusive conditions.
Thus the invention provides particularly well suited materials, which--when combined with the appropriate test protocol--allow significantly improved differentiation of absorbent articles.